This invention relates to a noise-canceling apparatus and, more particularly, to a noise-canceling apparatus capable of canceling noise at a prescribed position (observation point) in an automotive vehicle so that pleasant audio can be heard.
A known method of dealing with noise involves using a sound-absorbing material (this is a method of passive control). With a method that relies upon use of a sound-absorbing material, however, forming a silent area of little noise is troublesome and low-pitched sounds are not eliminated effectively. In particular, when noise within the passenger compartment of an automotive vehicle is prevented by passive control, the vehicle is increased in weight and the elimination of noise cannot be performed effectively.
For this reason, active-control methods in which a noise-canceling sound whose phase is the opposite of the noise is emitted from a speaker so as to reduce the noise have become the focus of attention and these methods are being put into practical use in factory and office interiors. Systems for reducing noise by active control have been proposed for the passenger compartments of automotive vehicles as well.
FIG. 9 is a block diagram of an apparatus for achieving the cancellation of sound. As shown in FIG. 9, an engine 11 which is a source of noise has its rotational speed R sensed by an rpm sensor 12. The output R of the sensor 12 is applied to a reference-signal generator 13, which generates a sinusoidal signal having a fixed amplitude and a frequency that conforms to the rotational speed R of the engine 11. The sinusoidal signal serves as a reference signal x.sub.n. When an engine is a source of noise, the noise generated by rotation of the engine has periodicity (this is periodic noise) and the frequency of the noise is dependent upon the engine rotational speed. In the case of a four-cylinder engine, for example, the frequency of periodic noise generated within the passenger compartment is 20 Hz when the rotational speed is 600 rpm (=10 rps) and 200 Hz when the rotational speed is 6000 rpm (=100 rps). These are secondary harmonics of the engine speed. Accordingly, the reference-signal generator 13 stores the sinusoidal data in a ROM and generates the reference signal x.sub.n by reading out and delivering this data as necessary. The timing at which this data is read out and delivered is controlled in accordance with the engine rotational speed R so that the reference signal outputted will have a frequency conforming to the engine rotational speed R.
The reference signal x.sub.n generated by the reference-signal generator 13 is applied to a noise-canceling controller 14 as an input. Also fed into the controller 14 is an error signal e.sub.n, which is a composite-sound signal that is a synthesis of noise S.sub.n and a noise-canceling sound S.sub.c at a noise-canceling position (an observation point, such as a point in the vicinity of the ears of the driver) within the passenger compartment. The noise-canceling controller 14 outputs a noise-canceling signal N.sub.c by executing adaptive signal processing so as to minimize the error signal e.sub.n. The controller 14 includes an adaptive signal processor 14a, an adaptive filter 14b constructed as a digital filter, a DA converter 14c for converting the output of the adaptive filter 14b into the noise-canceling signal N.sub.c, which is an analog quantity, and a filter 14d for producing a filtered-X signal (a reference signal r.sub.n for signal processing) by superimposing, on the reference signal x.sub.n, the propagation characteristic of a canceling-sound propagation system (secondary-sound propagation system) 18 extending from a speaker to the noise-canceling point.
A power amplifier 15 amplifies the noise-canceling signal N.sub.c and applies the amplified signal to a canceling speaker 16, which emits the noise-canceling sound S.sub.c. An error microphone 17 is disposed at the noise-canceling point so as to detect the aforesaid composite-sound signal, which is a synthesis of the noise S.sub.n and the noise-canceling sound S.sub.c, and output a composite-sound signal as the error signal e.sub.n.
The error signal e.sub.n at the noise-canceling point and the filtered-X signal r.sub.n, which is produced by the filter 14d, enter the adaptive signal processor 14a, which decides the coefficients of the adaptive filter 14b by using these two signals to execute adaptive signal processing in such a manner that the noise at the noise-canceling point is canceled out. For example, the adaptive signal processor 14a decides the coefficients of the adaptive filter 14b in accordance with a well-known filtered-X LMS (least mean square) algorithm so as to minimize the error signal en that has entered from the error microphone 17. In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b subjects the reference signal x.sub.n to digital filtering processing so that the DA converter 14c will deliver the sound-canceling signal N.sub.c. It should be noted that the reference signal x.sub.n must be a signal having a high correlation with respect to the noise S.sub.c to be canceled; sounds having no correlation with the reference signal are not canceled out.
When the engine 11 rotates, its rotational speed R is sensed by the rpm sensor 12, the reference-signal generator 13 generates the reference signal x.sub.n [see (a) in FIG. 10], whose frequency conforms to the engine rotational speed R, and the reference signal x.sub.n enters the noise-canceling controller 14. At this time the periodic engine sound (periodic noise) generated by the engine 11 reaches the noise-canceling point upon propagating through space having a noise propagating system (a primary-noise propagating system) that exhibits a prescribed transfer function. Accordingly, the noise (engine sound) S.sub.n at the noise-canceling point has a slightly lower level and a slight delay, as illustrated at (b) in FIG. 10.
Initially, the noise-canceling controller 14 produces the noise-canceling signal N.sub.c so as to have a phase opposite that of the reference signal x.sub.n, as a result of which the canceling speaker 16 outputs the canceling sound S.sub.c shown at (c) in FIG. 10, by way of example. However, since the level and phase of the noise S.sub.n are displaced somewhat from the level and phase of the canceling sound S.sub.c, the noise is not canceled out by the canceling sound S.sub.c and, hence, the error signal en is generated. The noise-canceling controller 14 decides the coefficients of the adaptive filter 14b by performing adaptive signal processing in such a manner that the error signal e.sub.n is minimized. In an ideal case, the phase of the canceling sound S.sub.c will be opposite that of the noise S.sub.n and the levels thereof will be in agreement, as shown at (d) in FIG. 10, so that the noise is canceled out.
In order simplify the description, the foregoing example deals with one noise source, one source (the speaker) for generating the canceling sound, and one noise-canceling point (the observation point). In actuality, however, there is more than one noise source and more than point (observation point) at which noise is desired to be canceled. In such case, more than one speaker is necessary since noise at a plurality of points cannot be canceled with only one speaker. FIG. 11 is a block diagram of a conventional noise-canceling apparatus for a case in which there are K-number of noise sources, M-number of speakers and L-number of observation points.
Numeral 21 denotes a noise-canceling controller (which corresponds to the noise-canceling controller 14 in FIG. 9) that operates so as to cancel out noise at each of a number of observation points. Numeral 22 denotes a primary-sound hypothetical propagation system (noise propagation system), which expresses systems along which noise is propagated from each noise source (not shown) to each observation point. Numeral 23 represents a secondary-sound propagation system (noise-canceling sound propagation system), which expresses systems along which canceling sound is propagated from each speaker to each observation point. The system 23 includes the characteristics of the speakers (not shown). Numeral 24 designates a signal synthesizer, which implements the function of a microphone at each observation point. The signal synthesizer 24 includes adders 24.sub.1 .about.24.sub.1 ' corresponding to a microphone at a first observation point, adders 24.sub.2 .about.24.sub.2 ' corresponding to a microphone at a second observation point, . . . , and adders 24.sub.L .about.24.sub.L ' corresponding to a microphone at an L-th observation point. Further, d.sub.d1n .about.d.sub.dLn represent external noise that is not the object of cancellation at each of the observation points.
The noise-canceling controller 21 includes a multiple-input/multiple-output adaptive filter (hereinafter referred to simply as an adaptive filter) 21a for inputting noise-canceling signals y.sub.a1n .about.y.sub.aMn to the speakers upon being provided with inputs of reference signals x.sub.a1n .about.x.sub.aKn (outputted by a reference-signal generator, not shown) conforming to the noise components generated by the noise sources, a filtered-X signal producing filter 21b, which is fabricated using the elements (propagation elements) of a transfer-function matrix of the secondary-sound propagation system 23, this filter being provided with inputs of the reference signals x.sub.a1n .about.x.sub.aKn conforming to the noise generated by the noise sources, and an adaptive signal processor 21c, which is provided with inputs of error signals e.sub.1n .about.e.sub.Ln prevailing at the observation points and filtered-X signals r.sub.111n .about.r.sub.LMKn outputted by the filter 21b, for deciding the coefficients of the adaptive filter 21a by executing adaptive signal processing using these input signals so as to cancel out the noise at each observation point.
FIGS. 12A and 12B are diagrams for describing the primary-sound hypothetical propagation system 22. The noise generated by K-number of noise sources N.sub.G1 .about.NG.sub.K reaches microphones (MIC.sub.1 .about.MIC.sub.L), which are provided at the respective observation points, upon propagating through the primary-sound propagation system 22 having prescribed frequency and phase characteristics. Accordingly, if we let H.sub.ji represent the transfer characteristic of a propagation system in which noise from an i-th noise source NG.sub.i reaches a j-th microphone MIC.sub.j, the primary-noise hypothetical propagation system 22 will be expressed as shown in FIG. 12B and the transfer-function matrix (H) thereof will be as follows: ##EQU1##
Each element H.sub.ij of the transfer-function matrix (H) is implemented by a FIR-type digital filter shown in FIG. 13. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients h.sub.0, h.sub.1, h.sub.2, . . . , and adders AD for adding the outputs of the multipliers.
FIGS. 14A, 14B are views for describing the secondary-noise propagation system 23. As shown in FIG. 14A, noise-canceling sounds generated by speakers SP.sub.1 .about.SP.sub.M arrive at the microphones MIC.sub.1 .about.MIC.sub.L, which are provided at the respective observation points, upon propagating through the secondary propagation system 23 having prescribed frequency and phase characteristics. Accordingly, if we let C.sub.ji represent the transfer characteristic of a secondary-noise propagation system in which a canceling sound based upon an i-th noise-canceling signal y.sub.ain reaches the j-th microphone MIC.sub.j, the secondary-noise propagation system 23 will have the form of the model shown in FIG. 14B and the transfer-function matrix (C) thereof will be as follows: ##EQU2##
Each element of the transfer-function matrix (C) is implemented by a FIR-type digital filter shown in FIG. 13, just as in the case of the primary-sound hypothetical propagation system 22. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients c.sub.0, c.sub.1, c.sub.2, . . . , and adders AD for adding the outputs of the multipliers.
FIG. 15 is a block diagram showing the filtered-X signal-producing filter 21b fabricated using each element C.sub.ij of the transfer-function matrix (C) of the secondary-sound propagation system 23.
The adaptive signal processor 21c updates the coefficients of the adaptive filter 21a by executing adaptive signal processing based upon the reference signals x.sub.a1n .about.x.sub.aKn and the signals e.sub.1n .about.e.sub.Ln that are a composite of the noise and canceling sounds at each of the observation points, and the adaptive filter 21a, to which the reference signals x.sub.a1n -x.sub.aKn are applied as inputs, generates the noise-canceling signals y.sub.a1n .about.y.sub.aMn and applies these signals to the speakers to cancel out the sound at each observation point.
The noise-canceling signals y.sub.a1n .about.y.sub.aMn outputted by the adaptive filter 21a do not reach the observation points as is. Rather, they reach the observation points upon being influenced by the frequency and phase characteristics of the secondary-sound propagation system 23. As a consequence, the adaptive signal processor 21c performs highly sophisticated noise-canceling control not by using the reference signals x.sub.a1n .about.x.sub.aKn as is but by employing a filtered-X LMS (multiple-error filtered X LMS, referred to as an "MEFX LMS") algorithm, which uses signals obtained by impressing the characteristics of the secondary-sound propagation system 23 on the reference signals. In other words, on the basis of the filtered-X LMS algorithm, the adaptive signal processor 21c updates the coefficients of the adaptive filter 21a using signals r.sub.111n .about.r.sub.LMKn, which are result of filtering the reference signals x.sub.a1n .about.x.sub.aKn by the filter 21b, and the composite-sound signals (error signals) e.sub.1n .about.e.sub.Ln at the observation points.
In FIG. 15, C.sub.ij represents a FIR-type digital filter for realizing each element C.sub.ij (see FIG. 14) of the transfer-function matrix (C) in the secondary-sound propagation system 23. The filter 21b is adapted so as to output the filtered-X signals r.sub.111n .about.r.sub.LMKn upon impressing all of the propagation elements upon each of the reference signals x.sub.a1n .about.x.sub.aKn (i.e., passing each reference signals through filters corresponding to all of the propagation elements). More specifically, the propagation elements C.sub.11 .about.C.sub.L1 from the first speaker to all of the observation points are made to act upon the reference signal x.sub.a1n to produce the filtered-X signals r.sub.111n .about.r.sub.L11n, the propagation elements C.sub.12 .about.C.sub.L2 from the second speaker to all of the observation points are made to act upon the reference signal x.sub.a1n to produce the filtered-X signals r.sub.121n .about.r.sub.L21n, . . . , and the propagation elements C.sub.1M .about.C.sub.LM from the M-th speaker to all of the observation points are made to act upon the reference signal x.sub.a1n to produce the filtered-X signals r.sub.1M1n .about.r.sub.LM1n. All of the propagation elements are made to act upon each of the reference signals x.sub.a2n, x.sub.a3n, . . . x.sub. aKn in a similar manner. This may be expressed as follows: EQU R.sub.11 =(r.sub.111n, r.sub.211n, . . . r.sub.L11n) EQU R.sub.21 =(r.sub.121n, r.sub.221n, . . . r.sub.L21n) . . . R.sub.M1 =(r.sub.1M1n, r.sub.2M1n, . . . r.sub.LM1n) . . . R.sub.MK =(r.sub.1MKn, r.sub.2MKn, . . . r.sub.LMKn)
FIG. 16 is a block diagram showing the multiple-input/multiple-output adaptive filter 21a, which has a structure similar to that of the primary-sound hypothetical propagation system 22 or secondary-sound propagation system 23. FIR-type digital filters are shown at A.sub.11n .about.A.sub.MKn. By way of example, each of these filters may be realized by delay elements DL.sub.1, DL.sub.2 . . . for successively delaying the input signal by one sampling period, multipliers ML.sub.1, ML.sub.2, ML.sub.3 . . . for multiplying each delay-element output by coefficients a.sub.0, a.sub.1, a.sub.2 . . . , and adders AD.sub.1, AD.sub.2 . . . for adding the multiplier outputs. The number of delay stages is limited to two.
The noise-canceling signal y.sub.a1n inputted to the first speaker is obtained by inputting the reference signals x.sub.a1n .about.x.sub.aKn to the digital filters A.sub.11n .about.A.sub.1Kn and then adding, the noise-canceling signal y.sub.a2n inputted to the second speaker is obtained by inputting the reference signals x.sub.a1n .about.x.sub.aKn to the digital filters A.sub.21n .about.A.sub.2Kn and then adding, . . . , and the noise-canceling signal y.sub.aMn inputted to the M-th speaker is obtained by inputting the reference signals x.sub.a1n .about.x.sub.aKn to the digital filters A.sub.M1n .about.A.sub.MKn and then adding.
When each of the FIR-type digital filters A.sub.11n .about.A.sub.MKn in the adaptive filter 21a is composed of three coefficients (two delay stages), the adaptive signal processor 21c decides the values of the coefficients by executing adaptive signal processing for each of the three coefficients of the FIR-type digital filters A.sub.11n .about.A.sub.MKn. That is, the adaptive signal processor decides coefficients a.sub.0, a.sub.1, a.sub.2 by performing the following operation with regard to these coefficients a.sub.0, a.sub.1, a.sub.2 of one FIR-type digital filter A.sub.ijn : ##EQU3##
In Equation (1), (n) signifies the value at the present sampling time, (n-1) the value one sampling earlier, (n-1) the value two samplings earlier, and (n+1) the value from the present time to the next sampling time. Accordingly, R.sub.ij (n-2) signifies the output of the filter 21b that conforms to the reference signal two samplings earlier, R.sub.ij (n-1) signifies the output of the filter that conforms to the reference signal one sampling earlier, and R.sub.ij (n) signifies the output of the filter that conforms to the reference signal at the present time. Further, .mu. represents a constant (step-size parameter) of less than 1, and e.sub.n represents the signal (error signal) that is the composite of the noise and canceling sound at each of the L-number of observation points.
In accordance with this noise-canceling apparatus, the adaptive signal processor 21c decides the coefficients of the FIR-type digital filters A.sub.11n .about.A.sub.MKn, which constitute the adaptive filter 21a, by executing adaptive signal processing based upon the filtered-X signals r.sub.111n .about.r.sub.LMKn, which are outputted by the filter 21b, and the composite-sound signals (error signals) e.sub.1n .about.e.sub.Ln that are a composite of the noise and canceling sounds at each of the observation points. The adaptive filter 21a, to which the reference signals x.sub.a1n .about.x.sub.aKn are applied, generates the noise-canceling signals y.sub.a1n .about.y.sub.aMn and applies these signals to the speakers SP.sub.1 .about.SP.sub.M (FIG. 14). Each speaker generates a canceling sound to cancel out the noise at each observation point.
FIG. 17 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there are one noise source (K=1), two speakers (M=2) and two observation points, i.e., two microphones (L=2). Numeral 21a denotes the adaptive filter, which is composed of two FIR-type digital filters A.sub.11n, A.sub.21n, numeral 21b denotes the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C.sub.11, C.sub.21, C.sub.12, C.sub.22 of the transfer-function matrix of the secondary propagation system, numerals 21c-1, 21c-2 denote adaptive signal processors (MEFX LMS) for deciding the coefficients of each of the digital filters in the adaptive filter 21a, SP.sub.1, SP.sub.2 represent speakers, and MC.sub.1, MC.sub.2 designate microphones disposed at the observation points.
FIG. 18 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there are one noise source (K=1), four speakers (M=4) and four observation points, i.e., four microphones (L=4). Numeral 21a denotes the adaptive filter, which is composed of four FIR-type digital filters A.sub.11n, A.sub.21n, A.sub.12n, A.sub.22n, numeral 21b denotes the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C.sub.11, C.sub.21, C.sub.31, C.sub.41 . . . , C.sub.44 of the transfer-function matrix of the secondary propagation system, numerals 21c-1 through 21c-4 denote adaptive signal processors (MEFX LMS), SP.sub.1 .about.SP.sub.4 represent speakers, and MC.sub.1 .about.MC.sub.4 designate microphones disposed at the observation points.
The frequency characteristics, inclusive of the speaker characteristics, of the secondary propagation system from the speakers to each observation point are not flat but vary as a function of frequency. FIG. 19 is a characteristic diagram showing the characteristics of speaker frequency. A frequency characteristic up to a noise frequency of 200 Hz, which corresponds to an engine rotational speed of 6000 rpm (=100 rps), varies approximately linearly in conformity with frequency. The frequency characteristic of the secondary-sound propagation system 23, which is the result of adding the frequency characteristic within the passenger compartment to this speaker characteristic, varies in conformity with frequency.
If the frequency of the noise to be canceled is constant, the coefficient convergence characteristic of the adaptive filter that relies upon adaptive signal processing is improved so that the coefficient values of the adaptive filter quickly converge to their optimum values. As a result, a satisfactory noise-canceling effect is capable of being achieved.
However, the frequency of the noise to be canceled fluctuates from one moment to the next. For example, the engine frequency fluctuates from one moment to the next and in dependence upon vehicle velocity, and the frequency of the engine sound also varies. When the frequency of noise fluctuates, gain varies in accordance with the frequency characteristic of the secondary-sound propagation system 23, and the coefficient convergence characteristic of the adaptive filter that relies upon adaptive signal processing deteriorates (i.e., there is a decline in the follow-up capability). The result is that the noise-canceling effect cannot be manifested satisfactorily. More specifically, in the adaptive signal processor, processing for deciding adaptive filter coefficients that conform to the present frequency characteristic (gain) of the secondary-sound propagation system is executed. However, when the frequency characteristic (gain) fluctuates at the next point in time, the coefficients that have been decided do not take on appropriate values that conform to the frequency characteristic at this next point in time and the coefficients of the adaptive filter do not converge quickly. This causes a decline in the follow-up capability.